Recently, small and light-weight portable appliances have been widely used, and lithium ion secondary batteries used as the power source of such appliances are required to have excellent safety and reliability, as well as high energy density.
When a sharp conductive object, for example, a nail, pierces a lithium ion battery, or when a conductive foreign matter, for example, large iron particles are undesirably included in a lithium ion battery in a production process thereof, the battery shorts out internally, thereby becoming overheated due to its high energy density. In the case of using singly a microporous thin film sheet made of polyolefin, such as polyethylene, as a separator in consideration of only the electrolyte-retaining ability, a problem occurs since the microporous thin film sheet shrinks due to heat at relatively low temperatures. That is, such heat shrinkage expands the internally shorted portion and induces further overheating.
As a technique to improve safety against short-circuiting, there has been proposed a method of forming a porous film on an electrode (see Patent document 1). Also, there has been a proposal to improve discharge capacity by using a porous film as an electrolyte-retaining layer (see Patent document 2).
Patent document 1: Japanese Laid-Open Patent Publication No. Hei 7-220759
Patent document 2: Japanese Laid-Open Patent Publication No. 2002-8730
Using a prior art porous film in combination with a microporous thin film sheet can suppress overheating due to internal short-circuiting. However, it cannot suppress overheating due to overcharge beyond design capacity.
Overcharge is detailed below. In the event of a failure of a charging circuit, the charging of a battery is not finished even if its design capacity is exceeded, thereby causing an overcharge reaction. Due to the overcharge reaction, lithium is excessively released from the positive electrode active material. In the case of LiCoO2, in particular, the crystal structure of the active material is destroyed and a large heat is generated, so that the whole battery is significantly overheated.
In order to suppress overheating due to overcharge, mainly two methods are available. One is a method of closing the pores of a separator by melting at a relatively low temperature, to eliminate its ionic conductivity. The other is a method of intentionally forming a partially short-circuited portion between positive and negative electrodes, to replace a seemingly overcharge current with a short-circuit current. The latter method is carried out by utilizing deposition of a conductive chemical species, specifically, lithium needle-like crystal (dendrite) deposited on the negative electrode, or transition metal deposited from the positive electrode active material on the negative electrode.
However, a conventional porous film is uniformly formed on the flat surface of the electrode active material layer, and the electrode reactivity therefore becomes uniform. Although this is a preferable mode in the ordinary charge/discharge range, it is not so in terms of suppressing overcharge reaction. Since the formation of a short-circuited portion by a conductive deposit occurs after the overcharge reaction has proceeded to a considerable extent in the whole electrode, it is impossible to suppress overheating.